Suspensions for automobile bodies include those having hydraulic rams arranged near respective wheels instead of spring dampers whereby the rams are driven by hydraulic pressure from a pressure accumulator to stably control the spatial orientation of the automobile bodies; in other words, to perform stable leveling control of the automobile bodies. Hydraulic fluids for suspensions of the above type are required to have as their basic properties desirable viscosity at high temperatures and preferable fluidity at low temperatures, namely, good viscosity-temperature characteristics in view of the severe use environments of automobiles.
Known conventional hydraulic fluids for automobile suspensions include those containing as a base oil a low-viscosity mineral oil having excellent low-temperature fluidity and as a viscosity index improver a large amount of polymethacrylate (PMA) to impart desired viscosity-temperature characteristics. Although such conventional hydraulic fluids based on mineral oil have good viscosity-temperature characteristics, they are insufficient in those other characteristics required.
In the conventional hydraulic fluids based on mineral oil, a mineral oil having a kinematic viscosity of about 2 mm.sup.2 /s (=cSt) at 100.degree. C. is used as a base oil and a large amount of PMA is added as a viscosity index improver to impart desired viscosity-temperature characteristics, so that the viscosity at 100.degree. C. is adjusted to about 6 mm.sup.2 /s or so. These hydraulic fluids have such problems as will be described below.
(1) In contrast to ordinary hydraulic fluids for automobiles--for example, ATF (automatic transmission fluid) and PSF (power steering fluid) which have a flash point of at least 200.degree. C.--the flash points of the above-described hydraulic fluids are very low, i.e., 100.degree.-140.degree. C. PA0 (2) Evaporation loss from ATF or PSF is practically 0%, whereas hydraulic fluids have an evaporation loss as high as 15-35%. PA0 (3) Anti-seizure property is low because of the low viscosity of the base oil. PA0 (4) Hydraulic fluids undergo significant viscosity reduction by shear force, so that their shear stability is poor. This means that hydraulic fluids are reduced in viscosity by shear force generated by the sliding parts of piston pumps or in the nozzles of shock absorber pistons. This viscosity reduction can be attributed primarily to the low shear stability of PMA.
As has been described above, it is extremely difficult for conventional mineral-oil-based hydraulic fluids to satisfy the various characteristics required for hydraulic fluids for automobile suspensions, in particular, to stably maintain good hydraulic response characteristics over a long period of time.